The computer industry is moving toward fast, packetized, serial input/output (I/O) bus architectures, in which computing hosts and peripherals are linked by a switching network, commonly referred to as a switching fabric. A number of architectures of this type have been proposed, culminating in the “InfiniBand™” (IB) architecture, which has been advanced by a consortium led by a group of industry leaders (including Intel, Sun Microsystems, Hewlett Packard, IBM, Compaq, Dell and Microsoft). The IB architecture is described in detail in the InfiniBand Architecture Specification, Release 1.0 (October, 2000), which is incorporated herein by reference. This document is available from the InfiniBand Trade Association at www.infinibandta.org.
A host processor (or host) connects to the IB fabric via a network interface adapter, which is referred to in IB parlance as a host channel adapter (HCA). Client processes running on the host communicate with the transport layer of the IB fabric by manipulating a transport service instance, known as a “queue pair” (QP), made up of a send work queue and a receive work queue. The IB specification permits the HCA to allocate as many as 16 million (224) QPs, each with a distinct queue pair number (QPN). A given client may open and use multiple QPs simultaneously. To send and receive communications over the network, the client initiates work requests (WRs), which causes work items, called work queue elements (WQEs), to be placed in the appropriate queues. The channel adapter then executes the work items, so as to communicate with the corresponding QP of the channel adapter at the other end of the link.
The QP that initiates a particular operation, i.e. injects a message into the fabric, is referred to as the requester, while the QP that receives the message is referred to as the responder. An IB operation is defined to include a request message generated by the requester and, as appropriate, its corresponding response generated by the responder. (Not all request messages have responses.) Each message consists of one or more IB packets. Typically, a given HCA will serve simultaneously both as a requester, transmitting requests and receiving responses on behalf of local clients, and as a responder, receiving requests from other channel adapters and returning responses accordingly. Request messages include, inter alia, remote direct memory access (RDMA) write and send requests and atomic read-modify-write operations, all of which cause the responder to write data to a memory address at its own end of the link, and RDMA read requests, which cause the responder to read data from a memory address and return it to the requester. Most response messages consist of a single acknowledgment packet, except for RDMA read responses, which may contain up to 231 bytes of data, depending on the data range specified in the request.
Although IB does not explicitly define quality of service (QoS) levels, it provides mechanisms that can be used to support a range of different classes of service on the network. Each IB packet carries a Service Level (SL) attribute, indicated by a corresponding SL field in the packet header, which permits the packet to be transported at one of 16 service levels. The definition and purpose of each service level is not specified by the IB standard. Rather, it is left as a fabric administration policy, to be determined between each node and the subnet to which it belongs.
Different service levels can be mapped to different virtual lanes (VLs), which provide a mechanism for creating multiple virtual links within a single physical link. A virtual lane represents a set of transmit and receive buffers in a network port. The port maintains separate flow control over each VL, so that excessive traffic on one VL does not block traffic on another VL. The actual VLs that a port uses are configurable, and can be set based on the SL field in the packet. As a packet traverses the fabric, its SL determines which VL will be used on each link.